1. Field of the Invention
The present invention relates generally to an optical semiconductor device, in which a semiconductor laser and an optical modulator or the like or integrating those. More specifically, the invention relates to an optical semiconductor device having a carrier recombination layer.
2. Description of the Prior Art
In a field of an optical communication in subscriber loop system, a semiconductor laser which does not require a temperature adjustment by a Peltier element or the like and auto power control (APC), has been desired for lowering of cost. In order to obtain a high optical output semiconductor laser used in a field of optical communication, it is required to have a structure which is difficult to turn on a pnpn current blocking layer of a thyristor structure upon high temperature or high current application. As a laser which can prevent the pnpn current blocking layer from turning on by charge-up, it has been known as a RIB-PBH (Recombination layer Inserted Blocking-Planar Buried Heterostructure) laser, in which a carrier recombination layer of InGaAsP, for example, is inserted in the blocking layer (T. Terakado, et al., "Extremely Low Thresholds 1.3 .mu.m Strained MQW Lasers with Novel p-substrate Buried-Heterostructure (RIBPBH) Grown by MOVPE Using TBA and TBP", 14.sup.th Laser Conference, paper PD9, (1994)).
However, in fabrication process of RIB-PBH laser, semiconductor etching process is required. Thus, such fabrication process merely achieves low uniformity and reproductivity.
In the recent years, as a laser which can improve uniformity and reproductivity with making semiconductor etching process unnecessary, "MQW BH-LDs with current blocking structure fabricated MOVPE and novel self alignment process" has been disclosed (Y. Sakata, et al., "Improved performance of MQW BH-LDs with current blocking structure fabricated MOVPE and novel self alignment process", Tech. Dig. of IOOC '95, paper FB2-3, (1995)).
FIG. 1 is a section showing a structure of the conventional MQW (Multi-Quantum Well) BH-LDs. A fabrication process of this laser diode will be discussed hereinafter. At first, a p-InP buffer layer 2 is formed on a p-type InP substrate having surface of (100) plane. Then, a silicon dioxide layer having an opening portion extending in 011! orientation is formed on the surface of the p-InP buffer layer 2 in a width of 1.5 .mu.m. A p-InP clad layer 4, a SCH-strained MQW layer 5 and an n-InP clad layer 10, for example, are selectively grown on the surface of exposed p-InP buffer layer 2 with taking the silicon dioxide layer as a growth blocking mask. Hereinafter, the portion consisted of p-InP clad layer 4, the SCH-strained MQW layer 5 and the n-InP clad layer 10 will be referred to as a selective growth portion.
FIG. 2 is a diagrammatic illustration showing an energy-band structure of the SCH-strained MQW layer. In FIG. 2, the upper solid line represents edge 21 of conductivity band, and the lower solid line represents edge 22 of valence band. A first guide layer 6 of InGaAsP is formed on the surface of the p-InP clad layer 4. On the surface, five 0.7% compression strained InGaAsP well layers 7 are formed. Four non-strained InGaAsP barrier layers 8 having a band gap wavelength 1.13 .mu.M are formed between well layers 7. Accordingly, an electron level in the well layer 7 is lower than the electron level of the barrier layer 8, and a hole level 24 in the well layer 7 becomes higher than the hole level of the barrier layer 8.
Also, a second guide layer consisted of InGaAsP is formed on the surface of the well layer 7. By these, SCH-strained MQW layer 5 is formed. The guide layers 6 and 9 have SCH (Separate Confinement Heterostructure) which are formed sandwiching the strained MQW layer, i.e. well layer 7 and the barrier layer 8. Accordingly, by the guide layers 6 and 9, a guided wave light in the strained MQW layer is confined.
The selective growth portion is surrounded by (100) plane and (111) B plane, which is in cross-sectionally trapezoidal configuration. Thereafter, a silicon dioxide layer is deposited on the entire surface. The thickness of the silicon dioxide layer at the (111) B plane of the selective growth portion becomes thinner than the thickness of the silicon dioxide layer at the (100) plane. Subsequently, the silicon dioxide layer at the (111) B layer is completely removed by performing etching over the entire surface. Then, the silicon dioxide layer is remained on the upper surface of the selective growth portion and on the surface of the buffer layer 2 where the selective growth portion is not formed.
Subsequently, in a form covering the selective growth portion, a positive resist having a width of 5 .mu.m is formed by photolithography. Then, etching is performed for the silicon dioxide layer with buffered hydrofluoric acid. By such etching, the silicon dioxide layer at the (100) plane on the p-InP buffer layer 2 can be removed completely by side etching. However, since the silicon dioxide layer is discontinuous at the (111) B plane in the selective growth portion, the silicon dioxide layer at the (100) plane on the selective growth portion can be maintained without effecting the side etching.
Then, a p-InP buried layer 12, an n-InP blocking layer 13 and a p-InP blocking layer 14 are sequentially grown on the surface of the p-InP buffer layer 2 with taking the silicon dioxide layer as a growth blocking mask. Subsequently, after removing the silicon dioxide layer on the selective growth portion (on the surface of the n-InP clad layer 10), an n-InP clad buried layer 17 and an n+-InGaAsP contact layer 18 are grown sequentially. Thereafter, a surface electrode 19 is formed on the contact layer 18 by vapor deposition or sputtering. In conjunction therewith, the back surface of the p-InP substrate 1 is polished to form a back side electrode 20 over the entire back surface. Thus, the conventional MQW BH-Lds is fabricated.
In the MQW BH-LDs thus formed, when a voltage is applied to the surface electrode 19 and the back surface electrode 20, electron and hole are coupled in the strained MQW layer (active layer) to be converted into a light. Then, a light in the strained MQW is guided by means of the guide layers 6 and 9, a light wave continuously oscillating from an end face can be obtained.
However, in the MQW BH-LDS, it is possible to cause turn on by charge-up of the blocking layer at high temperature or upon high current injection.